1. Field of the Invention
This invention relates to a seat belt system for a vehicle, such as an automotive vehicle, having an impact energy absorbing device which can be utilized for restricting an occupant restraining load exerted on a seat belt by the movement of an occupant when a vehicle is in a collision.
2. Description of the Prior Art
In one type of impact energy absorbing device of the prior art used with a seat belt system, the material for producing threads for weaving a seat belt or the manner in which threads are woven into a seat belt is varied so that the seat belt itself may be able to absorb the kinetic energy of an occupant of a vehicle when a collision occurs. Another type of such device known in the art includes a pipe connected to a seat belt and adapted to be expanded when a collision occurs, so that the impact energy can be absorbed by the expanded pipe.
In the aforesaid devices of the prior art, if an energy absorbing member is given a load of a predetermined value beforehand or the load is set at a value at which an occupant can be prevented from suffering injury when a collision occurs at a speed of say 50 mile/hour, the occupant would possibly be subjected to a shock of high intensity and suffer injury when a collision occurs at a speed of say 30 mile/hour. The fact that the set load value does not vary depending on the collision speed would naturally result in the amount of movement of the occupant in the space in the vehicle varying depending on the collision speed because the kinetic energy given to the occupant by a collision varies depending on the collision speed. Thus, the devices of the prior art have the disadvantage that, since the load of the energy absorbing member does not vary depending on the collision speed, it is impossible to effectively utilize the space in the vehicle.